Magnetization and Mossbauer study of the reentrant amorphous Fe₉₀Zr₁₀ alloy

Abstract

A detailed analysis of the magnetization and "zero-field" Mossbauer data taken on the amorphous Fe₉₀Zr₁₀ alloy in the temperature range 4.2-300 K reveals the following: (i) Spin-wave (SW) excitations at low temperatures, single-particle (SP) excitations and local-spin-density (LSD) fluctuations over a wide range of intermediate temperatures, and enhanced fluctuations in the local magnetization for temperatures close to the Curie temperature, T_C, contribute dominantly to the thermal demagnetization of spontaneous magnetization; (ii) SW modes soften at temperatures below the freezing temperature T_f, where long-range ferromagnetic order coexists with the cluster spin-glass order; (iii) the LSD fluctuations are completely suppressed when magnetic fields (H) higher than 5 kOe are applied and M(H,T) for H > 5 kOe is solely governed by the SW and SP excitations for temperatures up to 0.95T_C; (iv) contrary to some earlier claims, the spin-wave stiffness coefficient does not depend on H in the field range 5 kOe≤ H≤ 15 kOe; (v) the magnetic hyperfine-field distribution, P(Hhf), is bimodal and comprises two Gaussian distributions; (vi) the low-field spin fraction [ratio of the area under the low-field Gaussian curve to that under the P(H_hf)-vs-H_hf curve] grows at the expense of the high-field spin fraction as the temperature is raised above T≅ 150 K and amounts to about 90% of the total Fe spins for T≅ T_C; and (vii) the spin-freezing process does not start abruptly at T_f but proceeds gradually over a wide temperature range extending from 130 K (≅ 3T_f) down to 4.2 K. While the transverse spin freezing and the finite spin clusters (composed of antiferromagnetic Fe spins) plus ferromagnetic (FM) matrix models fail to explain some of our findings, the finite-FM-clusters-FM-matrix picture provides a satisfactory explanation for all the diverse aspects of the present results.